Stephen E. Creager

Inkjet-printed electrochromic devices utilizing polyaniline–silica and poly(3,4-ethylenedioxythiophene)–silica colloidal composite particles

Polyanline (PANI)–silica and poly(3,4-ethylenedioxythiophene) (PEDOT)–silica colloidal composite particles having a diameter of 200–300 nm were synthesized, then converted to intrinsically conducting polymer (ICP)-inks viasolvent exchange. These inks could be inkjet-printed on indium tin oxide-coated poly(ethylene terephthalate) films (ITO-PET film) using a commercial desktop inkjet printer. These PANI–silica or PEDOT–silica/ITO-PET assemblies were utilized as the active layer in electrochromic devices (ECDs). A number of devices were fabricated that exhibited 500 µm size patterning and utilized spatially controlled printing of the particles to allow for a dual image display. In addition, the PANI and PEDOT-based particles were blended into a single ICP ink that allowed for a wider absorption tuning range of the final ECD.

Solid Polymer Electrolytes from Polyanionic Lithium Salts Based on the LiTFSI Anion Structure

Effects of anion size on ionic conductivity were studied for a series of solid polymer electrolytes prepared from lithium polyanionic salts based on a series of lithium bis[(perfluoromethyl)sulfonyl]imide (LiTFSI) units connected together by perfluoroalkane linkers to form oligomeric anionic chains of variable length. Solid polymer electrolytes were prepared from the salts using polyethylene oxide as the host and characterized using X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy. Ionic conductivities were measured over a temperature range between 120°C and ambient for electrolytes with ethylene oxide (EO)/Li ratios of 30:1 and 10:1. Solid polymer electrolytes prepared from the lithium polyanionic salts exhibited ionic conductivities that were consistently lower (by factors of between 2 and 10) relative to those of monomeric LiTFSI-based electrolytes over the entire temperature and salt concentration ranges. This finding probably reflects a diminished contribution of anions to the overall conductivity for salts with large, polymeric anions. Trends in ionic conductivity with respect to anion chain length and EO/Li ratio were studied. The existence of an optimal anion chain length that is different for solid polymer electrolytes of differing EO/Li ratio was noted and is rationalized in terms of the cumulative effects of anion mobility, ion-pairing, variations in host chain dynamics in the vicinity of ions as a function of anion structure, and salt-phase segregation on the conductivity.

A Comparison of Bis[(perfluoroalkyl)sulfonyl]imide Ionomers and Perfluorosulfonic Acid Ionomers for Applications in PEM Fuel-Cell Technology

Two structurally related perfluorinated ionomer materials, one a conventional sulfonic-acid-batred ionomer (Nafion) and the other an experimental bis[(perfluoroalkyl)sulfonyl]imide-based ionomer in which the sulfonic acid group has been replaced by a sulfonyl imide acid group, were studied in parallel to evaluate their relative utility as membrane materials for use in polymer electrolyte membrane (PEM) fuel cells. Studies focused on membrane ionic conductivity and water content under varying conditions of relative humidity, and on device-level fuel-cell tests using membrane-electrode assemblies (MEAs) fabricated from membranes of the two ionomers. The overall finding is that the two ionomer materials behave similarly with respect to their electrochemical properties and performance in PEM fuel-cell devices. In one instance, a sulfonyl-imide-based MEA exhibited substantially improved performance relative to a comparable Nafion-based MEA in fuel-cell tests. The improvement is probably attributable to a combination of favorable materials properties and membrane thickness effects.

Solid polymer electrolytes from dilithium salts based on new bis((perfluoroalkyl)sulfonyl)diimide dianions. Preparation and electrical characterization

Abstract Solid polymer electrolytes (SPEs) were prepared from a series of dilithium salts based on new bis[(perfluoroalkyl)sulfonyl]diimide dianions using poly(ethylene oxide) (PEO) as the polymer host. SPE characterization was accomplished using thermal methods, powder X-ray diffraction, proton nuclear magnetic resonance ( 1 H-NMR) and electrochemical impedance spectroscopy (EIS). Ionic conductivities for SPEs made using the dilithium salts and also using the monomeric lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI) salt were measured over a temperature range between 120 °C and ambient for materials with EO/Li ratios of 30:1 (dilute) and 10:1 (concentrated). SPEs made using the dimeric salts exhibited ionic conductivities that were consistently low when compared with those from SPEs made using monomeric LiTFSI. This finding is thought to reflect a diminished contribution of the anions in the dimeric salts to the overall SPE conductivity. An unexpected finding of increasing ionic conductivity with increasing fluorine content in the dianions is thought to be the result of two opposing trends. One trend reflects an increase in anion size with increasing fluorine content, which diminishes anion transport and conductivity. Another trend reflects a decrease in anion basicity with increasing fluorine content that results in diminished ion pairing and an enhancement in the number of charge carriers, thereby increasing conductivity.

A Nernstian electron source model for the ac voltammetric response of a reversible surface redox reaction using large-amplitude ac voltages

A new model that predicts the reversible ac voltammetric peak profile of a surface redox reaction for an arbitrary choice of the ac voltage amplitude is described. The model is termed the Nernstian Electron Source (NES) model since it is based on a superposition of the fluctuating ac voltage onto a Nernstian distribution of states. The model extends previous theoretical treatments of ac voltammetry which were based on equivalent circuit models that are strictly valid only for small voltage perturbations. Two ferrocene-based monolayer systems were studied to test the predictions of the new model. The dependence of peak height on voltage amplitude for one such monolayer system deviated from the predictions of the older equivalent circuit model at modest amplitudes (Eac<25 mV) but was in excellent agreement with the predictions from the NES model at all measured amplitudes. The use of large-amplitude ac voltammetry for increasing signal amplitude and for driving otherwise slow surface redox reactions at appreciable rates is also demonstrated.

Electrochemical reactivity at redox-molecule-based nanoelectrode ensembles

Abstract A model describing electrochemical reactivity at nanoelectrode ensembles consisting of redox-molecule-based active sites immobilized on otherwise passivated electrode surfaces is presented. A mathematical treatment in terms of hemispherical diffusion of redox-active solutes to a layer of independent molecule-based nanoelectrode sites is shown to be equivalent to one in terms of a bimolecular diffusion-limited reaction between a layer of immobilized redox molecules and a reservoir of redox-active solutes. This equivalence derives from the fact that in both cases the mass-transfer problem is essentially that of hemispherical diffusion. The model is further developed to consider rate limitation by both the bimolecular redox reaction between the active-site molecule and redox molecules in solution and the heterogeneous redox reaction between the electrode and the active-site molecule. Analytical expressions are derived for the current–voltage relation corresponding to catalyzed electron transfer at an ensemble of redox-molecule-based nanoelectrode sites, and the expressions are used to interpret preliminary data for ultrasensitive electrochemical detection in flow streams via an electrochemical amplification process that is thought to involve redox mediation by individual analyte molecules adsorbed onto monolayer-coated electrodes.

Perfluoroalkyl Phosphonic and Phosphinic Acids as Proton Conductors for Anhydrous Proton‐Exchange Membranes

A study of proton-transport rates and mechanisms under anhydrous conditions using a series of acid model compounds, analogous to comb-branch perfluorinated ionomers functionalized with phosphonic, phosphinic, sulfonic, and carboxylic acid protogenic groups, is reported. Model compounds are characterized with respect to proton conductivity, viscosity, proton, and anion (conjugate base) self-diffusion coefficients, and Hammett acidity. The highest conductivities, and also the highest viscosities, are observed for the phosphonic and phosphinic acid model compounds. Arrhenius analysis of conductivity and viscosity for these two acids reveals much lower activation energies for ion transport than for viscous flow. Additionally, the proton self-diffusion coefficients are much higher than the conjugate-base self-diffusion coefficients for these two acids. Taken together, these data suggest that anhydrous proton transport in the phosphonic and phosphinic acid model compounds occurs primarily by a structure-diffusion, hopping-based mechanism rather than a vehicle mechanism. Further analysis of ionic conductivity and ion self-diffusion rates by using the Nernst-Einstein equation reveals that the phosphonic and phosphinic acid model compounds are relatively highly dissociated even under anhydrous conditions. In contrast, sulfonic and carboxylic acid-based systems exhibit relatively low degrees of dissociation under anhydrous conditions. These findings suggest that fluoroalkyl phosphonic and phosphinic acids are good candidates for further development as anhydrous, high-temperature proton conductors.

Ionomer Binders Can Improve Discharge Rate Capability in Lithium-Ion Battery Cathodes

A lithium-ion form of a perfluorosulfonate ionomer was used as a binder in LiFeP0 4 -based lithium-ion battery cathodes. Carbon-coated LiFeP0 4 and acetylene carbon black were blended with ionomer to prepare composite cathodes having a composition 60% LiFeP0 4 , 20% acetylene carbon black, and 20% binder by weight. Cathodes were tested against Li 4 Ti 5 O 12 anodes using 1.0 M and 0.1 M LiPF 6 -ethylene carbonate/diethyl carbonate (EC/DEC) electrolytes. Comparison was made with cathodes prepared using poly(vinylidene) difluoride (PVDF) as binder. At low discharge rates (e.g., C/5) both cathode types exhibited similar chargedischarge capacities and voltage profiles. However, under higher rate discharge conditions (e.g., > 1C, up to 5C) cathodes prepared using ionomer binder showed better discharge rate capability than cathodes having PVDF binder. This phenomenon was more pronounced when the salt concentration in the electrolyte was low (e.g., 0.1 M LiPF 6 -EC/DEC). These findings suggest that use of ionic binders can help to compensate for electrolyte depletion from the electrode porous space as lithium ions are intercalated into lithium-deficient LiFeP0 4 particles during rapid discharging. Potential uses for electrodes having ionomer binders in enabling lower cost battery electrolytes (because of the reduced need for salt) and in developing high rate cathodes that are nonporous or have low porosity are discussed.

Infrared Spectroscopy of Bis[(perfluoroalkyl)sulfonyl] Imide Ionomer Membrane Materials

Structural properties of the proton-exchanged forms of bis[(perfluoroalkyl)sulfonyl] imide (PFSI) ionomer materials were investigated. The hydration and dehydration of samples prepared as thin films and freestanding membrane were probed by applying transmission infrared spectroscopy. Spectral bands were assigned and effects of water incorporation into membrane pores and channels were understood by drawing upon results from related measurements performed on the structurally similar, perfluorosulfonic acid ionomer, Nafion. Both PFSI and Nafion membrane materials display a prominent infrared absorbance band near 1060 cm(-1) that arises from a vibrational mode of the ionizable group present on the side chains that extend from the poly(tetrafluoroethylene) backbone on the polymers. The mode can be traced to symmetric stretching of the -SO(3)(-) (sulfonate) group in Nafion and to antisymmetric S-N-S stretching within the sulfonyl imide end group (-SO(2)(N(-))SO(2)CF(3)) in the PFSI materials. For Nafion samples, the position and width of the band near 1060 cm(-1) are strongly sensitive to membrane hydration, whereas the band position and shape change only slightly during hydration and dehydration of PFSI materials. The possibility for greater charge delocalization over the sulfonyl imide moiety and shielding of hydrophilic species by the terminal -CF(3) group are suggested to explain the differences. These effects also likely influence the stretching modes of the side chain C-O-C groups. A pair of bands, sensitive to hydration and traceable to different C-O-C groups in a side chain, is present in the 970-990 cm(-1) region of Nafion. However, the two features are not well resolved and are less sensitive to hydration in spectra of PFSI samples. The most intense ionomer spectral bands arise from modes involving C-F stretching motion and appear between 1150 and 1250 cm(-1). Toward the high energy side of the envelope, there is substantial overlap with features of sulfonate group antisymmetric SO stretching modes in Nafion, but SO stretching modes of the sulfonyl imide moiety are higher in energy and better resolved in spectra of the PFSIs. During water uptake from a dry state into PFSI materials, a progression of features characteristic of solvated H(3)O(+) species appears across the water O-H stretching (2800-3800 cm(-1)) and H-O-H bending (1500-2000 cm(-1)) regions, similar to responses observed for water inside proton-exchanged Nafion.

Electrochemiluminescence-based detection of ferrocene derivatives at monolayer-coated electrodes

Abstract Luminol/hydrogen peroxide chemiluminescence catalyzed by electrochemically oxidized ferrocene derivatives was investigated at gold electrodes coated with alkanethiolate monolayers. The monolayer serves not only to suppress the direct luminol electrooxidation and consequent background chemiluminescence, but also to promote oxidation of certain ferrocene derivatives, which then act to catalyze the luminescent reaction between luminol and hydrogen peroxide. Chemiluminescence was particularly strongly promoted in a designed monolayer/catalyst system whereby attractive electrostatic interactions between a positively-charged ferrocene catalyst and a negatively-charged monolayer promote facile ferrocene oxidation, whereas repulsive interactions between the monolayer and the negatively-charged luminol molecule inhibit direct luminol oxidation. The ferrocene catalyst was detected both voltammetrically and by light emission in this system; however, detection by light emission exhibited a higher signal to background ratio than detection by oxidative current at comparable ferrocene concentrations.